This invention pertains to electro-optical position-measuring apparatus, and more particularly to circuitry usable with a linear photodetector array employed in such apparatus to determine the geometric mid-point of a spot of illumination cast onto the array.
A preferred embodiment of the invention is described herein in conjunction with determining the position of a log in a plywood mill, prior to the log being fed to a veneer peeler.
There are a number of applications, one of which has just been mentioned, where it is desired to locate the position of an object, or more specifically the position of a surface of the object, in space. Electro-optical systems have been proposed which are usable to perform this function, and at least one is known to employ what is referred to as a linear photodetector array. In this known system, a laser beam is directed toward the object of interest, and an optical system projects a reflected image of the point of impingement of the beam and the object onto the array. The system is arranged whereby the location of the imaged reflection along the array is directly interpretable to indicate the position of the impinged surface of the object.
Linear photodetector arrays are also used in a number of other kinds of apparatus wherein images, of one form or another, are projected onto the array.
In a substantial number of such applications, using an array of the type indicated, it is intended to determine as accurately as possible the geometric mid-point of the imaged spot of illumination on the array. Determination of this midpoint, obviously, is necessary to obtain maximum accuracy in whatever positioning measurement is being made.
A general object of the present invention is to provide unique circuitry for processing an output signal produced by such an array to determine the geometric mid-point of a spot of illumination thereon with an extremely high degree of accuracy.
Yet another object of the invention is to provide such circuitry which is relatively simple and inexpensive, yet highly reliable.
Still a further object of the invention is to provide such circuitry which can be used easily with substantially any available photodetector array, and in substantially any application where such an array is used in the manner generally described above.
According to a preferred embodiment of the invention, the circuitry thereof responds to the usual pulse-like signal produced during each scan cycle for such an array. As is well understood by those skilled in the art, such a signal is made up of what are known as individual "pixils"--one of these being produced for each illuminated photodiode in the array--and each pixil being characterized by a discreet DC voltage level. Near the "center of the pulse," it is probable that the voltage levels of most pixils will be substantially the same. The opposite edges of the pulse are derived from those pixils on the fringes of the spot of illumination, and will normally show distinctly different discreet levels. Also, a pulse derived from such an array will normally be asymmetric.
According to the invention, the output pulse thus provided by an array is filtered to smooth it, and is then compared with a DC reference voltage for the purpose, ultimately, of generating a square-wave pulse whose length depends upon the time that the level of the filtered pulse exceeds that of the reference voltage.
Included in the circuitry of the invention is a clock-pulse generator having at least two outputs, on one of which it provides clock pulses at one frequency, and on the other of which it provides clock pulses at another frequency which is one-half the first-mentioned frequency. During each scan cycle for the array, pulses at the higher frequency are counted during the time span beginning with the initiation of the cycle and ending with the initiation of a square-wave pulse (just above mentioned). Thereafter, pulses at the lower frequency are counted throughout the duration of the square-wave pulse. No further counting during a scan cycle occurs.
The total count so obtained during a scan cycle is directly readable to indicate the geometric mid-point of the spot of illumination on the array.
With such circuitry, an extremely accurate mid-point determination can be made. For example, a typical linear array might include, for each inch of length of the array, 1300 side-by-side photodiodes. Each such photodiode, when illuminated, produces what has been referred to above as a pixil in the output signal from the array--with the level of the pixil dependent upon the level of illumination on the particular diode, and the time-width of the pixil dependent upon the scanning rate. Utilizing the circuitry of the invention to determine spot illumination mid-point, this determination can be made within about one-tenth of a pixil width.
As will shortly become apparent, the circuitry proposed herein is extremely simple and can be assembled in very compact form.
These and other objects and advantages which are attained by the invention will become more fully apparent as the description which now follows is read in conjunction with the accompanying drawings.